Do not have enough SS to unlock currently:
Steel Champion is still at 52/80 so don't have him yet. Was only looking to farm him from WoG.
Mechanical Barbie is also 50/80 (will be 60/80 once login rewards end on 22nd)
Doctor Electronic (0/10)
and of course no Starship Commander either.

Another question - with the above list, is it likely I may have better odds using a non-Machine hero as a tank for example?

I got 3* difficulty 9 with Barbie, TimeSoldier, Machinist, Zombie Cyborg, and Mechanical Destroyer. Same team, got 2* on 10, 1* on 11. I am still working on leveling up and training Machinist, ZC, and MD. (I'm team lvl 110.)

Decided to go with Zombie Cyborg, Time Soldier, Machinist, Musketeer, and Puppet Master. Reset arena up to even spend 200 coins on a refresh to get Cyborg to 3* and took Musketeer to 4*. Then went "all in" of sorts, and used my resources to gear to take them all to 4/5 R2 (except Time Soldier, he's still R1).

Being able to use Musketeer's ultimate at beginning of rounds 2 and 3 was crucial for getting 3*. Took more retries than I care to admit but finally got there after lots of luck. I only managed to max Cyborg's 3rd and 4th skills too, 1st and 2nd were only at 16 and 5 respectively.